1. Field of the Invention
The present invention relates to a thermal printing apparatus wherein printing is effected on a planar heat-sensitive recording medium, by heating and thereby coloring a thermally fusible heat-sensitive material on the medium, and more particularly to a control arrangement for protecting a print head having heat-generating elements from sticking to the recording medium.
2. Discussion of the Prior Art
For printing desired images on a heat-sensitive recording medium, there is known a thermal line printer with a print head having an array of heat-generating elements arranged along a line of printing. In this thermal line printer, printing takes place with the heat-generating elements energized while the elements are held in contact with a surface of the recording medium, and while the recording medium is incrementally fed in a direction perpendicular to the line of printing, by a suitable drive source. Where an open-loop controlled stepping motor is used as the feed drive source, the feeding rate of the heat-sensitive recording medium may be easily controlled over a relatively wide range, without a speed reduction gear arrangement.
The heat-sensitive recording medium uses a thermally fusible heat-sensitive material which produces a color or colors when the material is heated and melted or fused by the heat-generating elements of the print head. When the heated heat-sensitive material is cooled and solidified, the heat-generating elements tend to stick to the material. When the feeding rate of the medium is relatively high, the medium may be smoothly fed without sticking of the print head to the medium surface, since the print head is in contact with the heated heat-sensitive material while the material is still in a sufficiently molten or less tacky state. When the medium feeding rate is relatively low, however, the print head is likely to stick to the heated heat-sensitive material, since the material has been considerably cooled and solidified before a relative feeding movement between the print head and the medium is initiated. Namely, the feeding increment with respect to the size of the heat-generating elements in the feeding direction is extremely small when the line feeding is effected at a low rate. Further, there exists a non-feeding time during which the heat-generating elements are kept in contact with the same local portions of the recording medium. Accordingly, the once heated heat-sensitive material is considerably solidified in contact with the heat-generating elements, before the medium is incrementally fed.
The sticking tendency of the print head indicated above causes an irregular feeding of the medium, wherein a certain number of first stepping pulses applied to the stepping motor do not cause a feeding movement of the medium, due to the sticking of the print head to the medium surface. That is, such stepping pulses are absorbed or accommodated by a power transmission system which includes a belt, for example. The feeding of the medium is initiated suddenly after the drive force exerted from the power transmission system overcomes the adhesive force between the print head and the medium surface. Thus, the known thermal line printer suffers from an irregular feeding involving unfavorable noises due to the sticking of the print head to the heat-sensitive medium.
A sticking tendency of the print head is also encountered in a thermal line printer of a type wherein the heat-generating elements of the print head are divided into a plurality of groups, so that the individual groups are sequentially operated to sequentially print corresponding divisions of a print line. This arrangement reduces a required capacity of a power source for the print head.
In the above type of thermal line printer, the heat-sensitive medium is fed with the print head held in contact with the medium surface, after the energization of the last group of heat-generating elements.
Since a considerable time is required for the sequential energization of all the groups of heat-generating element, the local portions of the heat-sensitive medium heated by the first and second groups, for example, have been solidified and kept in contact with the corresponding heat-generating elements. As a result, the initially heated portions of the medium tend to stick to the print head.
Thus, the sticking problem is also experienced in the above type of thermal line printer.
A sticking problem of the print head occurs for another reason. Namely, the print head tends to stick to a heat-sensitive recording medium, when printing is effected in two or more colors. Where printing is effected in black and red colors on a white background surface of the heat-sensitive medium, for example, the medium has a substrate on which are formed, a red-coloring heat-sensitive layer for producing red color when heated, an erasing heat-sensitive layer for erasing black color when heated, and a black-coloring heat-sensitive layer for producing black color when heated. These layers are superposed on each other on the substrate, in the order of description.
When a given amount of thermal energy is applied to the above multi-color heat-sensitive medium, only the black-coloring layer becomes active to produce black dots in the heated local portions of the medium. By increasing the thermal energy to a certain level, the erasing layer reacts with the blackened black-coloring layer, so as to erase the produced black dots. With the thermal energy further increased, the red-coloring layer becomes active to produce red dots in the heated portions of the medium.
Thus, the desired images may be printed in the black and red colors, by suitably controlling the amount of heat applied to the appropriate local portions of the medium, or by controlling the amount of an electric current applied to the corresponding heat-generating elements of the print head.
Since the formation of the red dots by heating the red-coloring layer requires a thermal energy which is several times as large as that required for producing the black dots. Therefore, the heat-generating elements should be continuously energized for a considerably long time to produce the red dots. This continuous energization may lead to shortening the life expectancy of the heat-generating elements.
In view of the above drawback, the red-coloring layer is heated intermittently, with the heat-generating elements intermittently energized several times, at a given time interval. In this case, each energization of the heat-generating elements to produce the red dots continues for a relatively short period, but the thermal energy is accumulated and increases to a level sufficient to produce the red dots, without deteriorating the heat-generating elements.
In the above case, the black dots of the relevant print line are first produced, and then the red dots of the line are produced. Alternatively, the energizations of the heat-generating elements corresponding to the black and red dots are initiated at the same time, and the heat-generating elements corresponding to the black dots are deenergized, before the elements corresponding to the red dots are deenergized, that is, before the printing operation of the relevant print line is completed. In the latter arrangement, the printing efficiency is improved, since the heat-generating elements corresponding to both of the black and red dots are simultaneously energized for a certain portion of the entire printing time.
In either of the above two arrangements for energizing the heat-generating elements to produce the black and red dots, however, the local portions of the medium to be colored black or heated by the heat-generating elements corresponding to the black dots have been considerably solidified before the red dots are eventually produced In other words, the heat-generating elements corresponding to the black dots tend to stick to the heated portions of the heat-sensitive medium, while being kept in contact with the medium surface for a relatively long time. Thus, the sticking problem also occurs when a multi-color printing operation is effected, irrespective of whether the heat-generating elements of the print head are divided into a plurality of sequentially energized groups, or not.